To examine structure-function relationships in biological membranes, presently incompletely understood, we will continue to study phagocytosis. This biological event is accompanied by profound alterations in the membranes of both the phagocyte and the ingested microorganisms. We will specifically continue the characterization and purification of a potent bactericidal factor of polymorphonuclear leukocytes, which in highly purified form also exhibits phospholipase A2 and bacterial envelope permeability increasing activities. BIBLIOGRAPHIC REFERENCES: Weiss, J., Schmeidler, K., Beckerdite-Quagliata, S., Franson, R.C. and Elsbach, P. Reversible envelope effects during and after killing of Escherichia coli by a highly-purified rabbit polymorphonuclear leukocyte fraction. Biochim. Biophys. Acta (Biomembranes) 436:154, 1976. Van Houte, A.J., Elsbach, P., Verkleij, A. and Weiss, J. Killing of Escherichia coli by a granulocyte fraction occurs without recognizable ultrastructural alterations in the bacterial envelope, as studied by freeze-fracture electron microscopy. Inf. Immun. 15:556, 1977.